This invention relates to electro-optic apparatus for making dynamic measurements of the angular deflection of cannon gun tubes, hereafter referred to as a Dynamic Muzzle Reference System (DMRS).
It is known in the art to sense the deflection of a cannon mounted on a tank (or like piece of artillery) and to compensate, and correct, for errors resulting therefrom to ensure that the cannon is aimed at the desired target; see for example, U.S. Pat. No. 4,665,795, titled Gun Muzzle Reference System, and U.S. Pat. No. 5,513,000, titled Autocollimator issued to Stephen R. Smith and John L. Lowrance (co-inventors of the instant application) and assigned to the assignee of the present application, and whose teachings are incorporated herein by reference.
Existing military tanks employ a manual Muzzle Reference System (MRS) mounted near the gun tube muzzle. The MRS includes a back-lit crosshair and collimating optics that is viewed by the gunner, through the Gunners Primary Sight (GPS), to manually align the GPS coordinates with the muzzle coordinates. This manual, static measurement requires the muzzle to be at a specific elevation angle in order to be visible to the turret mounted GPS. As such, the MRS presently used on tanks do not allow dynamic measurement of the muzzle to trunnion angle variations that result from tank motion and changes in ambient temperature, etc. (The term “trunnion” as used herein refers to the mounting mechanism-structure attached to the breech end of the gun tube that controls the elevation angle of the cannon's gun tube relative to the tank coordinates.)
Due to these limitations, tanks are being designed such that in the future they will employ a Dynamic Muzzle Reference System (DMRS) where DMRS, as used herein and in the appended claims, includes a system that can continuously and/or selectively measure changes in muzzle to trunnion angle, independent of elevation angle, including while the tank is in motion and when the cannon is being fired and/or when stationary.
Existing DMRS are based on autocollimator type instruments in which an optical collimator (transmitter-receiver=transceiver) mounted near the “breech” end of the gun tube (near the trunnion) projects a beam of collimated light to a mirror securely mounted near the muzzle end, as shown for example in U.S. Pat. No. 5,513,000. This mirror reflects the light beam back toward the transceiver where it is focused by the collimator optics or some other optics onto a position sensor. Changes in the angle of the muzzle mounted mirror result in the focused spot changing position on this position sensor. The electrical output of the photosensitive position sensor is a measure of the angular motion in azimuth and elevation of the muzzle relative to the trunnion.
As shown in FIG. 1, which depicts a prior art autocollimator based Dynamic Muzzle Reference System of the type shown in U.S. Pat. No. 5,513,000, the angular tilt of the muzzle mirror (M1) results in a lateral displacement (d) of the return beam at the trunnion which may be expressed as:d=(α)(2L)  eq. 1where:α is the change in muzzle tilt relative to the trunnion azimuth and elevation axes, in radians; and L is the distance from the muzzle to the trunnion.
The factor of 2 in eq. 1 is due to the fact that the distance traveled by the beam is 2L; that is, the mirror reflection process magnifies the tilt of the gun tube angle by a factor of 2. The optical collecting aperture of the transceiver must be large enough to subtend this lateral displacement, over the range of muzzle tilt angles of interest. The required opening (aperture) in the tank's armor for this collecting aperture decreases the effectiveness of the armor and also makes the DMRS itself more vulnerable to enemy fire.
Another deficiency of the existing muzzle mirror based DMRS system is that the light beam emitted from the transceiver in the direction of the muzzle is necessarily away from the tank increasing the risk of being detected by the enemy, even though most of the beam would be intercepted and reflected back by the mirror.
The deficiencies noted above are reduced in dynamic muzzle reference systems (DMRS) formed in accordance with the invention.